1. Field of the Invention
The present invention relates generally to the field of connecting rods and their bearing caps which transfer the pressure of combustion from reciprocating pistons to the rotary motion of the crank shaft and, more particularly, to connecting rods and their bearing caps which have been obtained by forging the rod and the cap as a single piece from a powder metal sintered preform and then separating them by cracking the cap from the rod.
2. Description of the Related Art
Connecting rods connect the reciprocating piston to the rotating crankshaft. The connecting rod assembly must be sturdy and complex to bear the load of combustion while undergoing the reciprocating and rotary motions. The assembly layout has three basic parts, a small end for connection to the piston, a big end for connection to the crankshaft and a beam connecting the ends. The small end has a cylindrical hole which receives a steel pin ("wrist" pin). The wrist pin is mounted in the opposed sides of the piston skirt. The wrist pin allows the rod to transfer the reciprocating motion of the piston to the rotary motion of the crankshaft.
The big end of the connecting rod assembly has a large cylindrical opening conforming to the shape of the journal of a crankshaft pin ("crank pin"). The large opening is provided by two separate pieces for enclosing the crank pin. The larger of the two pieces consists of the small end, the intermediate beam and a portion of the large cylindrical opening for the crank pin. The smaller of the two pieces is an end piece called the bearing cap which provides the remainder of the large cylindrical opening. The larger piece fits around one side of the crank pin. The bearing cap fits around the other side to enclose the crank pin. Bolts are used to connect the bearing cap to the larger portion of the connecting rod assembly to complete the connection of the piston to the crankshaft. Bearing inserts are used when mounting the assembly on the crank pin to allow the rod to move freely, thus the term "bearing cap."
A major design goal of the present invention is to make a cheaper, lighter connecting rod assembly without sacrificing strength and rigidity. This involves the efficient use of material without adversely affecting the performance of the connecting rod assembly. When creating a new rod, a key point about the conservation of mass is to use it economically. That is, the mass at any point should have a tight correlation to the load at that point.
It is well known to make connecting rods and their bearing caps by one of two ways: (1) forging the rod as a separate piece from the cap; or (2) forging the rod and the cap as a single piece and then separating them by cracking the cap from the rod. The first method is the traditional practice. The cap is forged in the direction of the longitudinal axis of the rod, i.e., radial to the cylindrical openings. This manufacturing method allows for a favorable cap shape from a strength and rigidity standpoint. However, weight control, dimensional stability and the number of machining steps are areas where this method may be improved.
The second method produces a connecting rod with good dimensional stability. The rod is forged in a direction normal to the longitudinal axis of the rod. One of the key benefits of this method is that no draft is needed to remove the part from the forging die. This allows for accurate weight control and provides locators for machining. The cap may be separated from the big end of the rod by fracturing the forging as is known in the art. This reduces the number of maching steps and provides unique mating surfaces on the cap and larger piece which resist lateral displacement when under load.
The rod and the cap may be made from traditional forging materials or powder metal material. Either of these materials may be used with either of the two forging processes. Powder metal has a distinct advantage over the traditional metals because a lighter weight product can be made when exactly the same design is used.
U.S. Pat. No. 2,053,962 to Lonas et al. discloses an example of the two-piece method for producing a connecting rod except that the larger piece is laminated, not forged. The shape of the bearing cap of Lonas et al. are similar to that of modem bearing caps in that there are two shoulders separated by an arcuate span conforming to a cylindrical portion of the shape of a crank pin. The outer arcuate surface of Lonas et al. have two mutually spaced circumferential ribs for purposes of reinforcement. The thickest portion of the rib is located at the center and the thinnest portion of the ribs is at the shoulders where the bolts seat.
Other connecting rods made by the two-piece method are known which have bearing cap shapes similar to that of Lonas et al. One particular design differs from the Lonas et al. design in that the area between the reinforcing ribs is filled in with material to create one thick reinforcing rib.
Still other known connecting rods made by the two-piece method have bearing caps with shapes differing from that of Lonas et al. One particular prior art design has a cap with an arcuate outer surface which is thinner at the center than at the ends.
U.S. Pat. No. 5,109,605 to Hoag et al. and U.S. Pat. No. 5,507,093 to Wittenstein et al. disclose examples of the one piece method for making connecting rods and bearing caps. In both instances, the outer arcuate surfaces of the bearing caps are not continuous. Rather, they are interrupted in the middle by a substantial rectangular lug. The bearing caps of Hoag et al. and Wittenstein et al. do not conserve as much mass as is possible.
Other examples of the one piece method for making connecting rods and bearing caps are disclosed in U.S. Pat. No. 5,536,089 to Weber et al. and U.S. Pat. No. 5,594,187 to Lynn. In these examples there are no centrally located rectangular lugs on the outer arcuate surface of the bearing cap. The arcuate spans appear to have uniform thicknesses. The bearing caps of Weber et al. and Lynn do not conserve as much mass as is possible.